The present invention relates to a method and apparatus for synchronising a mobile communications device to a first radio channel transmitted from a first radio transmitter when the mobile communication device is currently synchronised to a second radio channel transmitted from a second radio transmitter.
In a cellular radio telephone network, it is desirable in many circumstances to be able to at least approximately identify the position of a mobile station (MS), e.g. to within a few hundred metres. For example, this may be useful in order to despatch an emergency vehicle when an emergency call is made from the MS.
EP0767594 describes a system capable of achieving accurate position determination and which measures the time of arrival, at a MS, of synchronisation sequences transmitted at regular intervals by a set of respective neighbouring Base Transceiver Stations (BTSs) of the cellular network. The measured times of arrival (termed Observed Time Differences OTDs) are relative to the MS""s internal clock which is synchronised with synchronisation sequences transmitted by a serving BTS, user and signalling data being transmitted between the MS and the cellular network via the serving BTS. Assuming that the neighbouring BTSs and the serving BTS are transmitting their synchronisation sequences at exactly the same time, then the OTDs measured for the neighbouring BTSs represent the difference in propagation times between the respective neighbouring BTSs and the serving BTS. In practice, the neighbouring BTSs are not synchronised with the serving BTS (i.e. there exists a timing offset or a Real Time Difference RTD) and it is necessary to compensate the measured OTDs using the RTDs to obtain the actual or Geometric Time Difference (i.e. GTD=OTDxe2x88x92RTD). The principle of operation of the system of EP0767594 is illustrated in FIG. 1 where the GTD obtained for each neighbouring BTS can be used to define a hyperbola, the point of intersection of the hyperbolas being the location of the MS.
EP0767594 requires that the radio receiver of the MS be tuned in turn to each of the broadcast frequencies of the neighbouring BTSs for which an OTD is to be obtained. Once tuned, the received signal is analysed, e.g. using a matched filter, until the required synchronisation is observed. A problem with this approach is that if a synchronisation sequence is missed, e.g. due to noise, then the receiver must continue listening until the next synchronisation sequence is received. Acquisition of a synchronisation sequence may therefore take a relatively long time during much of which the receiver is needlessly occupied. Furthermore, because the MS is analysing large amounts of data between synchronisation sequences, the possibility of the detector mistakenly identifying a synchronisation sequence is greatly increased.
These same problems arise wherever it is necessary to synchronise a MS to a new radio channel, when the MS is currently synchronised with some other radio channel. For example, when a MS moves out of the coverage area of a BTS it is necessary to xe2x80x9chandoverxe2x80x9d the MS to a new BTS which can provide acceptable coverage to the MS. At any given time, a MS may make preliminary synchronisation measurements on several potential handover candidates so that a handover can be rapidly carried out when necessary.
It is an object of the present invention to overcome or at least mitigate the above noted disadvantages. This and other objects are achieved by notifying the MS of the expected time of arrival of a synchronisation sequence or feature transmitted from a BTS to which the MS is to be synchronised.
In a Time Division Multiple Access (TDMA) cellular radio telephone system, the present invention provides a method of synchronising a mobile communications device to a first radio channel transmitted from a first radio transmitter when the mobile communication device is currently synchronised to a second radio channel transmitted from a second radio transmitter, and wherein said first radio channel conveys at least one synchronisation feature, the method comprising:
determining the real time difference between the operating clocks of said first and second radio transmitters;
transmitting said determined real time difference, or a value or values derived therefrom, from said second transmitter to the mobile communications device, over said second radio channel; and
causing the mobile communication device to listen to said first radio channel for said synchronisation feature during a time window derived from said real time difference or said value(s).
Where the time window over which the mobile communications device listens to said first radio channel is relatively narrow, embodiments of the present invention may substantially reduce the possibility of noise or other signals on other portions of the first radio channel giving rise to erroneous detection results. In addition, embodiments of the present invention may reduce the power consumption requirements of the mobile device as the receiver of the device need be active only for the duration of the time window.
Preferably, said real time difference is determined using time delay relationships obtained for a radio receiver whose position is fixed and known. In one embodiment of the present invention, said step of determining the real time difference between the operating clocks of said first and second radio transmitters comprises:
causing a radio receiver, whose position is known and which is synchronised to said second radio channel, to listen on said first radio channel for said synchronisation feature and to determine the time of arrival (OTD) of the transmitted feature, where said time of arrival (OTD) represents the difference between the propagation delays over said first and second radio channels to the mobile device, offset by the real time difference (RTD) between the operating clocks of the two transmitters; and
determining the actual difference (GTD) between the propagation delays over said first and second radio channels to the mobile device using the known positions of said radio receiver and the first and second radio transmitters; and
determining said RTD using said OTD and said GTD.
The present invention may advantageously be used in a system arranged to determine the position of a mobile communications device and where the device is required to measure OTDs for a plurality of neighbouring base transceiver stations, relative to a serving base transceiver station. For each of the neighbouring base transceiver stations, the cellular network determines a RTD and transmits it to the mobile device. The mobile device then uses the RTDs to define time windows during which it listens to the respective radio channels. Preferably, the list of neighbouring base transceiver stations is transmitted to the mobile device from the serving base transceiver station, together with the respective RTDs.
The present invention may be used to determine a Timing Advance value for a new base transceiver station (TAN) to which a mobile device is to be xe2x80x9chanded overxe2x80x9d. This can be done prior to handover, using the Timing Advance value previously determined for the current, serving base transceiver station (TAS), the real time difference (RTD) for the new base transceiver station relative to the serving base transceiver station, and the observed time difference (OTD) measured for the new base transceiver station, i.e. TAN=2(OTDxe2x88x92RTD)+TAS.
Preferably, said synchronisation features are synchronisation sequences which are transmitted by a base transceiver station at regular intervals over a broadcast radio channel.